Well, I am not sure how to block them, but what you can do is write in the listing that you will cancel the bids of anyone with less then 5 feedbacks unless they contact you before bidding. You can then monitor the listing and cancel those bids if they do not contact you. Unfortunately due to the large numbers of NPB's on Ebay, some of which may be phishing for personal info, I can understand the need to do this and the frustration with a lack of response on Ebay's part.

Have you tried typing in " cancel bids" in the help section on Ebay. I always ask questions and they then give me a link on what do do, including cancelling bids.

ie copied and pasted from Ebay when asking how to cancel a bid.

How to cancel a bid
If you need to cancel a bid:
Go to Cancelling bids placed on your listing
- opens in new window or tab
.
Enter the item number, the username of the member whose bid you're cancelling, and the reason you're cancelling the bid.
Select cancel bid.
Or, you can also cancel a bid below.

Your welcome, now ebay has made so many changes its awlful, they relist your item if it doesn't sell. They take it upon themselfs to automatically relist it, then if you don't want to , you have to go and end the listing. you have no choice!. I think they are making the buy it now auctions for "Until Cancelled" you have no choice in that either.